Tires, conveyor belts and reinforced high pressure hoses are typical of but a few of the articles wherein cured and uncured components can be contiguously combined. Generally, the manufacture of these articles involves the assembly of a plurality of layers of fully compounded rubber that have been reinforced with carbon black and the like. Tires, as a more particular example, include components such as beads, sidewalls, carcasses, treads, and belts. In some instances, it may be desirable to cure one or more of these individual components off site and prior to assembly and vulcanization of the tire. An advantage of precuring is to impart integrity to the rubber based component so that it will resist distortion during subsequent building and assembly operations which, in turn, allows more precise alignment of components, greater accuracy during building and, at the end of assembly, improved tires. Also, because the bead and tread stock components have varying thicknesses, by subjecting some such structural components to precure, the cure time of the final product can be decreased. Moreover, the ability to combine vulcanized and unvulcanized rubber based components would permit a variety of articles to be manufactured, such as tires, utilizing one or more "standard" components or elements to which variable elements e.g., treads, can be bonded.
Accordingly, it has been desirable to precure or vulcanize certain components either partially or fully prior to overall assembly to produce the finished article. Unfortunately, however, the bond interface between contiguous components, one of which is vulcanized and one of which is unvulcanized, has not been acceptable. One manner of improving the adhesion calls for the mechanical buffing of the surface of the vulcanized rubber component, but this is an extra step and cannot always be employed.
The major hurdle to a broader application of this technique of employing precured together with uncured components has been the lower adhesion observed between the precured and the uncured compounds following their covulcanization. The art has attempted to address the issue of developing or improving the bonding between contiguous rubber layers but has not always been successful where the layers are cured and uncured.
U.S. Pat. No. 1,274,091, for example, discloses a composite sheet of vulcanized rubber comprising sheets of uncured rubber that have been washed and dried, one of which is broken down by passage through rolls and contains a vulcanized agent, while the other is neither broken down nor contains a vulcanizing agent. The two uncured sheets are ultimately covulcanized.
U.S. Pat. No. 1,402,872 provides a method for uniting masses of vulcanizable rubber by interposing a layer of rubber without sulfur therebetween and then covulcanizing the multilayer mass.
U.S. Pat. No. 1,434,892 provides a method of forming a sheet of rubber by combining one ply containing sulfur with a second ply containing an accelerator and thereafter covulcanizing the multilayer sheet.
U.S. Pat. No. 1,478,576 is directed toward sheet rubber patch materials for the repair of inflatable rubber articles. The material comprises a composite including a rubber layer containing a non-migratory accelerator and a rubber layer containing sulfur.
Other U.S. patents which teach the covulcanization of uncured rubber sheets each containing different vulcanizing agents and/or amounts thereof include U.S. Pat. Nos. 1,537,865, 1,537,866, 1,569,662, 1,777,960, and 2,206,441.
Thus, while others have covulcanized rubber sheets comprising different vulcanizing agents and amounts, the art has not provided a method for attaining good adhesion between contiguous rubber articles or components, one of which is vulcanized and one of which is unvulcanized. More particularly, the art has not recognized heretofore, the existence of gradient crosslink densities at the interface between cured and uncured rubber and hence, has not been able to provide good adhesion therebetween.
The use of irradiation to effect a partial cure of at least a portion of a tire component, other than the tread, followed by conventional cure of the tire is disclosed in U.S. Pat. Nos. 4,166,883 and 4,221,253 and 4,851,063, respective divisionals. The foregoing patents do not suggest the use of an interphase layer according to the present invention.